Process for the electrolytic deposition of niobium or tantalum



United States Patent 3,093,554 PROCESS FOR THE ELECTROLYTIC DEPOSITION OF NIOBIUM OR TANTALUM Fritz Kern, Binningen, Switzerland, assignor to Ciba Limited, Basel, Switzerland, a Swiss firm No Drawing. Filed May 27, 1960, Ser. No. 32,152 Claims priority, application Switzerland June 12, 1959 3 Claims. (Cl. 204-) This invention provides a process for the electrolytic deposition of the metal niobium or tantalum in a state of very fine particle size from a melt of an alkaline earth metal or alkali metal halide containing fluoride.

For many purposes it is desirable to have the metal niobium or tantalum available in a state of very fine particle size. Grinding these metals to obtain a very fine particle size is complicated and increases the cost of manufacture, especially as it is almost impossible to grind the metal itself, owing to its ductility, so that it is first hydrogenated, then ground in this brittle condition, and subsequently dehydrogenated. Grinding also involves the risk that the metal may take up during the grinding process undesired impurities, for example, hydrogen, oxygen etc. To carry out the grinding operation in an atmosphere of a protective gas in order to avoid such impurities would considerably complicate the process. It is, therefore, desirable to obtain the metal directly in the state of fineness which is required for subsequent use.

In the process for the electrolytic deposition of niobiurn and tantalum in accordance with this invention from a melt of an alkaline earth metal or alkali metal halide containing a fluoride, the vapor of a halide of the metal is introduced during the electrolysis in a quantity such that there is maintained in the melt an excess of the metal in relation to stoichiometric ratio of metal to fluorine required to form the double fluoride of the metal.

In order to form the double fluoride of the metal a molar ratio of fluorine to tantalum or niobium of 7:1 is required, but in the process of this invention a smaller molar ratio is used, and advantageously a molar ratio of 3:1. Accordingly, in the known process the metal has generally been evaporated in the form of its pentachloride in a melt containing an alkali metal or alkaline earth metal fluoride and the rate of evaporation has been so adjusted that the evaporated pentachloride is converted as completely as possible, into the double fluoride, that is to say, it is endeavoured to maintain as accurately as possible the stoichiometric ratio of metal to fluorine required to form the double fluoride. The metal obtained by that process mainly has a particle size that is too large for certain uses, especially for the manufacture of porous sinte-red bodies for condensers. In contradistinction thereto there are obtained by the process of this invention particle size distributions which consist primarily of the smaller particle sizes required for such uses.

The following example illustrates the invention:

Example The electrolytic cell was used consisting of a graphite crucible having a diameter of 8 centimeters and a height of 21 centimeters containing the melt and serving as anode. Centrally disposed within the anode was a cathode consisting of a nickel rod having a thickness of ice 1.5 centimeters and immersed in the melt to a depth of 10 centimeters. The whole cell was surrounded by a casing in which a protective atmosphere of argon was maintained. The graphite crucible contained 1,000 grams of an equimolecular mixture of sodium chloride and potassium chloride, which was maintained at 750 C. The melt also contained 10% by weight of tantalum (as TaCl and 2.1% by weight to fluorine (as KF), which corresponded to a ratio of tantalum to fluorine of 122.0.

The melt was electrolyzed and at the same time tantalum pentachloride vapor was introduced into the melt in such a quaitity that the concentration of tantalum and the tantalum-fluorine ratio remained constant in the melt. At a current density of 400 amperes the quantity of vapor introduced was about 700 grams per hour.

After one hour the electrolysis was discontinued, and the cathode was withdrawn from the melt. The deposited tantalum was removed from the cathode, freed from electrolyte by washing and divided by sieving into the particle size set out in the following table. The 260 grams of tantalum powder obtained in this manner had the following particle size distribution:

Particle size,

Percent 0 6 12 53 29 When working in the same apparatus with a somewhat higher fluoride content of 5.6% by weight, which corresponds to a tantalum: fluorine ratio of 1:5.3, there were obtained 320 grams of tantalum powder, which had the following distribution of particle sizes in the above ranges and therefore contain a considerably smaller proportion of fine particles.

Particle size, 1.1.

Percent 2 5 44 40 9 On the other hand, a control experiment carried out with the stoichiometric ratio of tantalum to fluorine of 1:7 had the following particle size distribution:

Particle size, p.

Percent 5 40 35 15 5 3 4 2. In the electrolytic production of tantalum from an is introduced and in which a molar ratio of tantalum to alkali halide-tantalum halide melt containing fluoride, fluorine is maintained in excess of 1:7 and at most 1:3.

introducing into the melt, wherein there is a stoichiometric excess of tantalum from the outset, vapor of tan- References Cited in the file of this Patent talum pentachloride and maintaining in said melt a 5 UNITED STATES PATENTS molar ratio of tantalum to fluorine greater than 1:7 and 1 874 090 Briggs Aug 30, 1932 at most 1:3, whereby there is a stoichiometric excess of 2899369 51min Aug. 11 1959 tantalum over that required to form the double fluoride 2:981:666 Kern Apr. 1961 of tantalum.

3. A pulverulent tantalum obtained by electrolytic 10 FOREIGN PATENTS deposition of tantalum from an alkali halide-tantalum 1,154,129 France Apr. 2, 1958 halide melt into which vapor of tantalum pentachloride 215,201 Australia May 22, 1958 

1. IN THE ELECTROLYTIC PRODUCTION OF TANTALUM FROM AN ALKALI HALIDE-TANTALUM HALIDE MELT CONTAINING FLUORIDE, INTRODUCING INTO THE MELT VAPOR OF TANTALUM PENTACHLORIDE AND MAINTAINING IN SAID MELT A MOLRA RATIO OF THANTALUM TO FLUORINE GREATER THAN 1:7 AND AT MOST 1:3, WHEREBY THERE IS A STOICHIOMETRIC EXCESS OF TANTALUM OVER THAT REQUIRED TO FORM THE DOUBLE FLUORIDE OF TANTALUM. 